Medical dosimetrists are allied health specialists that provide treatment planning in support of radiation oncology. The radiation oncology professional societies have recognized a critical shortage of medical dosimetrists in the United States. This shortage of trained specialists is stalling the implementation of technically advanced radiotherapy, in particular three-dimensional conformal radiation therapy and intensity-modulated radiation therapy, Clinical trials of radiation therapy in combination with new pharmaceutical and biological agents will not be possible without a strong medical dosimetry workforce. The majority of dosimetry students are mentored one to three students at a time in individual radiation oncology departments. Much of the training has become on-the-job apprenticeship without formal structure or clearly defined didactic goals. The program proposed here will produce and support a well-structured, nationally available, web-based computer-aided distant learning tool for the mentors of medical dosimetrists. Medical physicists, radiation oncologists, and medical dosimetrists from around the country have made commitments to serve as volunteer authors of the nearly two-dozen modules in the tool. Internet training specialists sited at Stanford University, an education consultant, and a psychometric consultant will support the development of the modules? content consistent with the recommendations of the American Association of Medical Dosimetrists. The distant learning modules will contain assessment instruments for both the summative evaluations of the students as well as the formative evaluation of the learning tools. The program will be developed with the aid of a series of annual workshops at which the Stanford developers will share analyses of the formative assessments with the authors, and set goals and priorities for continued development and refinement of the tool. Medical physicists, radiation oncologists, and medical dosimetrists in radiation oncology centers will continue to serve as local mentors for the students. The mentors will structure their own training programs and augment the distant learning tools with their own training methods. The mentors will develop structured clinical rotations to provide the students practical experience. A trial application of the tools will be conducted at the University of Wisconsin at La Crosse in a training program distributed over Wisconsin and neighboring states. Nationally 20 radiation oncology sites are committed to using the tool. The intent of this program is for graduates from these programs to apply for the certification examination administered by the Medical Dosimetrist Certification Board when they have accrued the required two years of training and experience.